1. Field of the Invention
The present invention is related to an oscillation circuit, particularly to an oscillation circuit for generating a driving signal which drives a vibrator, by using the signal obtained from the vibrator, that is used for a vibrating gyroscope which can detect a rotational angular velocity.
2. Description of the Prior Art
The vibrator 1 includes a regular triangular prism-shaped vibrating body 2, and on three side faces of the vibrating body 2, piezoelectric elements 3a, 3b and 3c are formed respectively. Each of the piezoelectric elements 3a, 3b and 3c has a piezoelectric layer, and electrodes are formed respectively on both surfaces of the piezoelectric layers . One of the electrodes is bonded to a side of the vibrating body 2 and the other is used for signal input/output. In the vibrator 1, when similar driving signals are applied to the two piezoelectric elements 3a and 3b or it is applied to the piezoelectric element 3c, the vibrating body 2 bends and vibrates in perpendicular direction to a main surface of the piezoelectric element 3c. When the vibrator 1 is not rotating, similar signals can be detected from the piezoelectric elements 3a and 3b. When a rotational angular velocity is applied to the vibrator 1 around a central axis of the vibrating body 2 , the bending and vibrating direction of the vibrating body 2 is changed by a Corioli's force and signals in proportion to the rotational angular velocity are detected from the two piezoelectric elements 3a and 3b. In this case, for example, a signal with larger level is detected from the piezoelectric element 3a and that with smaller level is detected from the other piezoelectric element 3b in response to the rotational angular velocity.
When using the vibrator 1 for the vibrating gyroscope, (1) the piezoelectric element 3c is connected to an input terminal of an oscillation circuit and an output terminal of the oscillation circuit is connected to the two piezoelectric elements 3a and 3b, (2) the two piezoelectric elements 3a and 3b are connected to the input terminal of the oscillation circuit and the output terminal of the oscillation circuit is connected to the piezoelectric element 3c, or (3) the piezoelectric element 3a or 3b is connected to the input terminal of the oscillation circuit and the output terminal of the oscillation circuit is connected to the piezoelectric element 3c in order to apply the driving signal to the vibrator 1. Further, the two piezoelectric elements 3a and 3b are connected to an inversion input terminal and a non-inversion input terminal of a differential amplifier circuit to detect the rotational angular velocity by a difference between detected signals from the two piezoelectric elements 3a and 3b.
FIG. 5 is a circuit diagram showing the conventional oscillation circuit for applying a driving signal to the vibrator 1 shown in FIG. 4. The oscillation circuit 10 has the input terminal 11 which is connected to the piezoelectric element 3c of the vibrator 1. The piezoelectric element 3c is grounded via a resistor 4c. The input terminal 11 is connected to an input terminal of an automatic gain control circuit 13 via a buffer 12 made of an operational amplifier. The automatic gain control circuit 13 consists of an operational amplifier 14, a resistor 15 for negative feedback, and a FET 16 as a variable impedance element; and its gain is reduced as the impedance between tile drain and the source of the FET 16 increases. Further, the output terminal of the buffer 12 is connected to the gate of the FET 16 incorporated in the automatic gain control circuit 13 via a control signal generator 17. The control signal generator 17 generates a control signal based on a difference between (1) the reference voltage V.sub.ref of a reference power supply 18 and (2) the signal which appears at the output terminal of the buffer 12, that is, the signal from the piezoelectric element 3c. Moreover, the output terminal of the automatic gain control circuit 13 is connected to the output terminal 20 via a phase-shifting circuit 19. The output terminal 20 is connected to the piezoelectric elements 3a and 3b via the resistors 4a and 4b, respectively.
In the oscillation circuit 10 shown in FIG. 5, the signal obtained from the piezoelectric element 3c of the vibrator 1 is applied to the piezoelectric elements 3a and 3b as the driving signal via the buffer 12 and automatic gain control circuit 13. When an amplitude of the signal obtained from the piezoelectric element 3c increases because an amplitude of the driving signal applied to the piezoelectric elements 3a and 3b increases, the voltage level of the control signal supplied to the gate of the FET 16 of the automatic gain control circuit 13 by the control signal generator 17 is decreased. Therefore, the impedance between the drain and the source of the FET 16 increases, the gain of the automatic gain control circuit 13 decreases, and so the amplitude of the driving signal applied to the piezoelectric elements 3a and 3b decreases. In reverse to this, when the amplitude of the signal obtained from the piezoelectric element 3c decreases because the amplitude of the driving signal applied to the piezoelectric elements 3a and 3b decreases, the voltage level of the control signal supplied to the gate of the FET 16 of the automatic gain control circuit 13 by the control signal generator 17 is increased. Therefore, the impedance between the drain and the source of the FET 16 decreases, the gain of the automatic gain control circuit 13 increases, and so the amplitude of the driving signal applied to the piezoelectric elements 3a and 3b increases. Consequently, in this oscillation circuit 10, it can be expected that a stable driving signal be applied to the piezoelectric elements 3a and 3b of the vibrator 1. Thus, when the stable driving signal is applied to the piezoelectric elements 3a and 3b of the vibrator 1, the variation of a sensitivity for detecting the rotational angular velocity is suppressed in the vibrating gyroscope in which the vibrator 1 is employed.
However, the voltage level applied between the drain and the source of the FET 16 used as a variable impedance element must be kept low in the oscillation circuit 10 shown in FIG. 5 in order to secure the linearity of the output signal to the input signal of the automatic gain control circuit 13. It is desirable that the voltage level be less than 100 mV, though it depends on the FET employed. Consequently, in order to cope with a large input signal for the automatic gain control circuit 13 and to attain a wide range in the gain of the automatic gain control circuit 13, the resistance value of the negative feedback resistor 15 must be great to obtain a high gain in the automatic gain control circuit 13; this results to deteriorate a signal-to-noise ratio of the driving signal.
Further, in the oscillation circuit 10 shown in FIG. 5, an electrostatic capacity between the drain and the source of the FET 16 changes according to the change of the impedance between the drain and the source of the FET 16 as the variable impedance element of the automatic gain control circuit 13. Consequently, a phase difference is produced between the input signal and the output signal of the automatic gain control circuit 13, and it changes in response to the change of the gain of the automatic gain control circuit 13.
In other words, the changes of states of circuit elements in the oscillation circuit 10 shown in FIG.5 affects the stability of oscillation, and so the driving signal applied to the piezoelectric elements 3a and 3b of the vibrator 1 fluctuates. This results in the fact that the signal detected from the piezoelectric elements 3a and 3b of the vibrator 1 is not stabilized and so the detection sensitivity of the rotational angular velocity is not stabilized either in the vibrating gyroscope wherein the vibrator 1 which is driven by the driving signal of the oscillation circuit 10 is employed.